Magnetic deflection mass spectrometer having two sectors with a spacing therebetween



4, 1970 s. GROSSEL T L 3,523,185

MAGNETIC DEFLECTION MASS SPECTROMBTER HAVING TWO SECTORS WITH A SPACING THEREBETWEEN Filed 001;. 25, 1967 PRIOR ART F I 6 3 /26 INVENTORS STANLEY GROSSEL WI LIAM LANKENAU ATTORNEYS United States Patent Oflice 3,523,185 Patented Aug. 4, 1970 3,523,185 MAGNETIC DEFLECTION MASS SPECTROMETER HAVING TWO SECTORS WITH A SPACING THEREBETWEEN Stanley Grossel, New York, and William Lankenau, Northport, N.Y., assignors to Veeco Instruments, Inc., Plainview, N.Y., a corporation of New York Filed Oct. 25, 1967, Ser. No. 677,973 Int. Cl. HOlj 39/34 US. Cl. 250-413 6 Claims ABSTRACT OF THE DISCLOSURE A mass spectrometer in which there is a single magnetic stage having two sectors with a spacing therebetween. The elements are arranged such that there is no focus of the ion beam between the sectors.

The present invention relates generally to mass spectrometry and in particular to an improved construction for mass spectrometers which significantly increases signal to noise ratio without the necessity for employing a two stage mass spectrometer arrangement and which provides other improved operational advantages.

In mass spectrometry as used for quantitatively determining the presence of ions of a given mass, it is known to use either single stage spectrometers or multiple stage 7 spectrometers. The single stage spectrometers typically comprise an ion source, an ion collector, an evacuated chamber therebetween and a single magnet whose field is arranged to act upon ions as they travel from the source to the collector, bending the ions through radii depending upon their particular masses. By proper alignment and positioning of the various elements and by use of a magnetic field of the proper strength, the collector measures the amount of ions of a selected mass. The signal to noise ratio associated with such units may be relatively low due to scattering which occurs at various locations between the source and the collector; this results in the collector receiving ions of masses other than the particular one of interest. In order to increase signal to noiseratio, it is known to use a two stage spectrometer in which ions focused at the point in a single stage device at which the collector is located are allowed to pass through that point and into a second stage. The second stage essentially is duplicative of the first stage and serves to decrease the noise. The collector is positioned at the focus point of the ion beam at the end of the second stage. Thus, stray ions which would have been measured by the collector at the interstage focus, if the unit had been a single stage device, are eliminated by the walls of the evacuated chamber in the second stage or by appropriate baffling, etc. Although the strength of the desired signal may be somewhat reduced because the ions must travel through a longer path, the strength of the noise is reduced by a far greater amount thus producing a materially enhanced signal to noise ratio.

A number of problems have been associated with the use of multiple stage mass spectrometers. Included among these, merely by way of example, are the problems produced by oil deposition on the walls, baflles and slits of such a device. When contamination of this type occurs, especially in the area of the small slit at the interstage focus, the edges of the slit become charged which produces materially decreased ultimate signal strength. Accordingly, a need has developed for an arrangement in which interstage slits can be eliminated. Furthermore, by their very nature, multiple stage units require relative large evacuated chambers requiring increased instrument sizes. There are other disadvantages in both single and multistage spectrometers which have produced a demand for an improved mass spectrometer.

It has been determined that a mass spectrometer with split magnet sectors rather than separate magnet stages offers improved characteristics. For purposes of this discussion, split sector magnets are those in which the ion beam between the sectors does not come to a focus as opposed to the situation in a conventional two stage mass spectrometer where the ion beam has a focus between the respective stages. Among the advantages of a split sector spectrometer is the fact that the design inherently eliminates any desirability for small interstage slits which have heretofore presented such extreme maintenance problem. Other advantages include a reduction in overall size of the evacuated chambers, a decrease in expense and a material increase in signal to noise ratio over the prior art single stage instruments.

Accordingly, it is an object of the present invention to provide a novel structure for a mass spectrometer oifering improved operations. Specifically, it is an object of the present invention to provide a single stage mass spectrometer which provides increased signal to noise ratio.

It is further within the contemplation of the present invention to provide a mass spectrometer construction which inherently allows for less frequent maintenance reducing the deleterious effects of contamination.

It is a further object of the present invention to provide a mass spectrometer construction in which an instrument having a high signal to noise ratio is of reduced size in comparison to prior art multistageinstruments.

In general, it is the object of the present invention to provide a mass spectrometer of increased operational life which requires less meticulous care than was needed for prior art instruments and which nevertheless produces improved high magnitude at reasonable manufacturing costs.

In accordance with the present invention there is provided a magnetic mass spectrometer which has been found to be particularly useful in the isolation and quantitative measurement of ions of asingle mass. This type of spectrometer finds useful employment in leak detectors.

A spectrometer in accordance with the present invention comprises a source of ions, an evacuated vessel in which ions from the source travel along a generally curved beam path, an ion collector for measuring ion current at the end of that beam path and two magnet sectors positioned at spaced locations along the beam path for bending ions in their travel from the source to the collector. Each of the magnet sectors is made up of a single magnet having a pair of pole pieces arranged on opposite sides of the beam path in order to produce a magnetic field between the poles which passes through the beam path in a direction perpendicular to the flow of ions.

In instances in which the entry of ions from the source into the magnetic field is perpendicular to the edge of the magnetic field, the location of the ion source, the collector and the apexes of the two magnet sectors will all lie substantially on the same line and the portion of the beam path between the two magnet sectors may be generally parallel to that line. Finallly, a spectrometer in accordance with this invention has means to collect ions of masses other than the one which is to be measured.

These means may comprise the walls of the evacuated chamber and appropriate bafiling means. It is a feature of this mass spectrometer that it does not have an intersector focus and, thus, does not use small slits in this region which in time becomes contaminated and requires maintainance.

The above brief description, as well as further objects, features and advantages of the present invention, will be best understood by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic view of a conventional prior art single stage mass spectrometer;

FIG. 2 is a schematic drawing of a mass spectrometer constructed in accordance with the present invention illustrating its relationship to and differences from the conventional single stage mass spectrometer illustrated in FIG. 1; and

FIG. 3 is a similar schematic view of a still further embodiment of the present invention and shows a mass spectrometer which differs in some respects from that shown in FIG. 2.

FIG. 1 is a simple schematic view of the prior art and shows a mass spectrometer, generally designated by the numeral which includes as its most basic elements a source of ions 12, a collector 14 and a magnet 16 positioned between the source 12 and the collector 14 for bending a beam of ions as they travel from the source 12 to the collector 14. The space between the source 12 and the collector 14 is encased within a gas tight envelope (not shown) which provides an evacuated environment for the beam path. The beam is schematically illustrated by three lines which indicate the central ray 18 and a pair of outer rays 20, 22 which designate the extremes of the width of the beams of ions of a given mass emanating from the source 12. By known principles, as the beam 18, 20, 22 travels under the influence of the magnetic field of the magnet 16, it is bent and ions of various masses are bent through different radii. The path shown in the drawing is of ions of the single mass of interest. Ions of higher atomic weight are bent through a longer radius and their beam ends below the collector 14. Thus, by proper placement of the collector 14, measurement of the ion level of a given atomic mass may be accomplished.

Mass spectrometers such as that illustrated schematically in FIG. 1 are useful in a large variety of analytical instruments and one of the most common uses is in leak detectors. If it is desired to test a vessel for leakage, it may be evacuated and then surrounded with, for example, helium. Samples taken from the interior of the vessel are then analyzed in a mass spectrometer such as that shown in FIG. 1 and, assuming the spectrometer has been designed to detect ions of mass four, the atomic weight of helium, the collector 14 will detect the presence and determine the amount of helium and thus determine the relative size of the leak, if any, in the vessel being investigated. Obviously, leaks of helium which produce a signal no greater than the noise which is present in such an instrument will not be detected.

It is known that noise can be reduced by use of a multistage device. However, these devices have many disadvantages. One of these is the difficulty in obtaining proper alignment of the various elements because of the precise nature of the locations of the various focal points. Since proper alignment is critical, the tolerances involved in the construction of such instruments is of a high order. It is desirable to produce a mass spectrometer which obtains the advantageous results of a multistage spectrometer but which does not include the inherent requirements for high tolerance construction and assiduous maintenance associated with such devices.

In mass spectrometers employing split sector magnets in accordance with the present invention, it is possible to vary, virtually at will, the distance between the sectors. Obviously, this is indicative of the materially decreased requirements for high tolerances as compared to prior art multistage instruments. Furthermore, and possibly more importantly, it offers the instrument designer the capacity to vary instrument design with greater ease. It also enables an increase in signal to noise ratio by simply increasing the distance between magnet sectors. Since ions leaving a magnet sector see a smaller solid angle as the distance between the two magnet sectors is increased,

an improvement in signal to noise ratio can be obtained simply by increasing the distance between the two magnet sectors.

FIG. 2 is a schematic illustration of a mass spectrometer constructed in accordance with the present invention. Comparison of the elements shown in FIG. 2 and in FIG. 1 clearly illustrate the difiierences between the two instruments. The mass spectrometer of FIG. 2 is generally designated by the numeral 50. It includes as its basic elements a source of ions 52, an ion collector 54 and a pair of magnet sectors 56, 58 arranged at spaced locations along the ion beam path. The magnet sectors cause a first bend and then a second bend in the path of ions as they travel from the ion source 52 to the ion collector 54.

In the illustrative showing of FIG. 2, the magnet sectors 56, 58 each have been illustrated to be geometrically and operationally one half of the magnet 16 in the spectrometer 10. In other words, a beam of ions from the source 52 enters the magnet sector 56 in the same manner as the beam from the source 12 entered the magnet 16 and, for each given distance into the field created by the magnet, the beam is bent the same amount. However, since the magnet 56 is one half of the magnet 16, i.e., the beam travels under the influence of the magnet 56 for half of the time and distance it travels under the influence of the magnet 16, the beam is bent one half of that amount and, as illustrated in FIG. 2, leaves the field of the magnet sector 56 having been bent through one half the amount of the beam passing through magnet 16. As shown in FIG. 2, the rays are generally parallel when they leave the magnet sector 56. For identification purposes, the ion beam between the two magnet sectors 56, 58 is identified by the designation 66, 68 and 70 relating to the central and two outer rays respectively.

It will be appreciated that the ion beam 66, 68 and 70 in the region between the two magnet sectors 56, 58 is substantially a beam of ions of a single mass, ions of other masses having been separated by the influence of the magnetic field of the magnet sector 56. Of course, there remains scattered ions of other masses within the region of the beam 66, 68, 70 resulting from ions scattered by collision with walls of the evacuated chamber and ions scattered by collision with random particles in the evacuated chamber. It is these scattered ions which the present invention attempts to prevent from reaching the ion collector 54.

The beam 66, 68, 70 travels further towards the collector 54 through a distance D between the magnet sectors 56, 58 and enters the influence of the magnetic field of the second magnet sector 58. In that field, the beam is bent once again and is formed into the converging beam shown in FIG. 2 as the central ray 72 and the two outer rays 74, 76. The magnetic field of the magnet sector 58 further separates ions of diflerent masses essentially purifying the nature of the beam 72, 74, 76 such that the ions focused on the collector 54 are of an increased order of mass uniformity. The beam at the collector 54 is of a higher order of purity than the beam at the collector 14 in the mass spectrometer 16, all other conditions being equal.

In this example, the ion source 52, the ion collector 54 and the apices of the magnet sectors 56, 58 all lie substantially on one straight line. Furthermore, the central ray of the beam between the magnet sectors 56, 58 (the beam designated by the numbers 66, 68, 70), is essentially parallel to this single straight line. It will be appreciated that in the mass spectrometer 50 there are no slits between the two magnet sectors 56, 58-the size of the beam in that region is large. Thus, the disadvantages of contamination of the interstage slit in a multistage spectrometer is eliminated in mass spectrometers in accordance with the present invention.

The distance D may be varied with relatively minor design changes and by increasing that distance the amount of noise received by the ion collector 54 will be decreased.

This is so because with increasing distance D, the collector 54 presents a smaller solid angle for receiving scattered ions leaving the magnet sector 56.

Bafiiing, of course, may be conveniently employed in the intersector region, however, such bathing bears little relation to the minute slits which are employed at the interstage focus of multistage spectrometers. The charge which builds up on contaminated slits has a great effect with small slits but is much less important on large bafiles.

Another mass spectrometer in accordancefwith the present invention is illustrated in FIG. 3 and is generally designated by the numeral 100. The mass spectrometer 100 comprises basically a source of ions 102, a'collector 104 and a pair of magnet sectors 106 and 108: arranged at spaced locations along the beam path of ions -traveling from the ion source102 to the ion collector 104. As in the previous embodiments, the entire beam path is contained within an evacuated chamber.

In the mass spectrometer 100, the two magnet sectors 106, 108 bend the beam of ions through a greater angle than either the single stage instrument 16 or the split magnet sector instrument 50. Emanating from the ion source 102 is a beam of ions at elevated energy which travels along a beam path defined by central ray 110 and the outer rays 112, 114. The beam in this region is comprised of ions of many different masses, however, under the influence of the magnetic field of the magnet sector 106, ions of mass four, for example, are bent through a precise radius and exit in a beam designated by the central ray 116, and the outer rays 118, 120. In this particular case, as with the mass spectrometer 50, the first sector 106 is designed to produce a parallel beam in the intersector region. However, this is not a requirement; the spectrometers are shown with this characteristic merely for simplicity of disclosure. The beam may be either convergent or divergent provided that there is no focus in the intersector region. The second sector 108 is equal in size and effect to the first sector 106 and causes the beam of ions of mass four to focus in a converging beam designated by the central ray 122 and the outer rays 124, 126 at the ion collector 104. It will be appreciated that the foregoing disclosures of two illustrative embodiments of the present invention have been presented in schematic form for simplicity of disclosure. In actual practice, design variations of both minor'and major nature may be made without departing from the spirit and scope of the present invention. A number of such design variations have been alluded to herein and still further variations are possible. Accordingly, the following claims should be interpreted broadly, consistent with the spirit and broad scope of this invention.

What we claim is:

1. A magnetic mass spectrometer for the isolation and quantitative measurement of ions of a single mass comprising a source of ions, an evacuated vessel in which ions from said source travel along a generally curved beam path, an ion collector for measuring ion current, said source of ions and said ion collector defining opposite ends of said beam path, a first and a second magnet sector alongsaid beam path for bending the path of ions from said source to said collector, each of said magnet sectors positioned at spaced locations along said beam path and creating a magnetic field perpendicular to the plane of the bend in said beam path and bending ions of said single mass in a first bend within the: first magnet sector arid in a second bend within said second magnet sector with the beam path between said sectors being free in allplanes from any focus of ions of} said single mass, said; source, said collector and said magnet sectors being arranged to focus a beam of ions of; said single mass on 'gsaid collector, and means for collecting diverse ions of masses other than said given mass and for preventing scattering by said diverse ions.

2. A mass spectrometer in accordance with claim 1 wherein said source, said collector and the apices of'said magnet sectors "are on a straight line and wherein, said means for collecting diverse ions include the walls of said evacuated vessel. 1

3. A mass spectrometer in accordance with claim 2 wherein the bea'ih path of ions of said given mass in the region between said magnet sectors is parallel to said straight line.

4. A mass spectrometer in accordance with claim 1 wherein said magnet sectors are permanent magnets.

5. A mass spectrometer in accordance with claim 1 wherein the distance of said beam path between said magnet sectors "is selected to reduce the number of ions of diverse masses which are not of said single mass which travel to said collector, said distance being independent of the respective distances between said source and said first magnet sector and said collector and said second magnet sector. a

6. A mass spectrometer comprising an evacuated vessel through which passes a beam path, a source of ions, an ion collector and two sector-shaped magnet means for deflecting the flow of ions in said beam path, each of said sector-shaped magnet means having apices, said source of ions, s'aid ion collector and said two apices lying on a straight line and said line being parallel to the beam path between said two sector-shaped magnet means of ions of the mass focused on said collector, said sectorshaped magnefimeans forming magnetic fields such that ions traveling from said source to said collector are focused at said collector and are not focused at other locations along said beam path.

References Cited UNITED STATES PATENTS 3,087,055 1963 Liebl. 3,138,706 611964 Brown et a1. 3,213,276 10/ 1965 Enge. 3,231,735 1*[1966 Peters.

WILLIAM F. LINDQUIST, Primary Examiner A. L. BIRCH, Assistant Examiner 

